|  | 3.2 | Segmentation with Switching Architectures
| 3.2.5 | How frames are used in VLANs
Switches are one of the core components of VLAN communications. Each switch has the intelligence to make filtering and forwarding decisions by frame, based on VLAN metrics defined by network managers. The switch can also communicate this information to other switches and routers within the network. The most common approaches for logically grouping users into distinct VLANs are frame filtering and frame identification (frame tagging). Both of these techniques look at the frame when it is either received or forwarded by the switch. Based on the set of rules defined by the administrator, these techniques determine where the frame is to be sent, filtered, or broadcast. These control mechanisms can be centrally administered (with network management software) and are easily implemented throughout the network. Frame filtering examines particular information about each frame. A filtering table is developed for each switch; this provides a high level of administrative control because it can examine many attributes of each frame. Depending on the sophistication of the LAN switch, you can group users based on a station's Media Access Control (MAC) addresses or network-layer protocol type. The switch compares the frames it filters with table entries, and it takes the appropriate action based on the entries. In their early days, VLANs were filter-based and they grouped users based on a filtering table. This model did not scale well because each frame had to be referenced to a filtering table. Frame tagging uniquely assigns a VLAN ID to each frame. The VLAN IDs are assigned to each VLAN in the switch configuration by the switch administrator. This technique was chosen by the Institute of Electrical and Electronic Engineers (IEEE) standards group because of its scalability. Frame tagging is gaining recognition as the standard trunking mechanism; in comparison to frame filtering, it can provide a more scalable solution to VLAN deployment that can be implemented campus-wide. IEEE 802.1q states that frame tagging is the way to implement VLANs. VLAN frame tagging is an approach that has been specifically developed for switched communications. Frame tagging places a unique identifier in the header of each frame as it is forwarded throughout the network backbone. The identifier is understood and examined by each switch prior to any broadcasts or transmissions to other switches, routers, or end-station devices. When the frame exits the network backbone, the switch removes the identifier before the frame is transmitted to the target end station. Frame identification functions at Layer 2 and requires little processing or administrative overhead.